The present invention relates to a keyboard assembly for providing electrical outputs corresponding to multiple keys, to signal utilization electronic devices such as typewriters and other data processing equipment.
In such a keyboard for electronic devices, a multiplicity of keys are disposed in plural rows to provide electric signals corresponding to the keys which have been depressed on their top faces. To improve ease of operation of the keys, attempts based on human engineering or ergonomics have been made to arrange the keys such that an operating surface generally defined by the top faces of the individual keys is curved to a downwardly convex shape in cross section taken along a line perpendicular to the rows of the keys. There have been proposed the following two methods to obtain such a curved operating surface of the keyboard.
These two methods will be described with reference to FIGS. 12 and 13.
The first method uses a curved key holder plate 2 having multiple guide holes 1, as shown in FIG. 12. The holder plate 2 is made from a steel plate by shaping it to a suitable curvature in the transverse cross section. In the guide holes 1, keystem guides 5 are fixedly inserted to slidably guide respective keystems 4 which carry at their upper ends keytops 3 having the finger-pressed top faces. In this case, the keys (3, 4) are all equally sized so that an operating surface 6 defined by the top faces is curved substantially to the curvature of the key holder plate 2.
In this method, however, it is required to fix the individual keystem guides 5 in the guide holes 1 formed in the shaped key holder plate 2. This assembling procedure is cumbersome and time-consuming, and reduces the efficiency of manufacture of the keyboard to an appreciable extent, and accordingly pushes up the cost of manufacture.
While the above method is advantageous in that the key holder plate 2, which is shaped under plastic working from a metal sheet, is capable of maintaining an initially given curved profile virtually permanently, the metal plate is required to be relatively thick for permanency of the original shape, and this inherently increases a total weight of the keyboard assembly, which may be considered as an undesired factor in the recent trend in the art toward providing compact and lightweight equipment.
The second method is illustrated in FIG. 13, wherein an upper casing 7 of a keyboard is provided with integrally formed keystem guides 8 which slidably support respective keystems 9 having keytops 10 fixed to their upper ends. In this method, an operating surface 11 of the keyboard is established by forming the keytops 10 in different sizes and shapes, depending upon the positions in which they are disposed. For example, the keytops 10 carried on the keystems 9 disposed in one of plural rows are formed with a top face having a curvature which is different from that of the keytops 10 carried on the keystems 9 in another of the plural rows.
Thus, the above second known method requires different kinds of keytops or keytops and keystems to provide different contours of top faces of the keys so that the top faces cooperate to form the curved operating surface 11. This means a need of using different kinds of molds for forming the different keys, and consequently an increased cost of manufacture of the keyboard assembly.
In the light of the above inconveniences of the known keyboards, an improved keyboard is proposed as disclosed in Japanese Patent Application No. 58-59365 filed on Apr. 20, 1983 (which has not been published at the time of filing the present application) in the name of one of the assignees of the present application. The corresponding U.S. patent application, Ser. No. 598,920 was filed on Apr. 10, 1984 now U.S. Pat. No. 4,528,428, and assigned to said one of the assignees of the present application. The keyboard as shown in the above U.S. patent application, includes a key holder plate of synthetic resin which has plural rows of holes and annular guide portions concentric with the holes so that plural rows of keys are carried movably through the holes and the guide portions. The key holder plate is disposed so as to close a rectangular aperture formed in an upper casing so that the keys extend through the aperture. Below the key holder plate, there is disposed a printed circuit board which includes a substrate carrying on its upper surface multiple pairs of stationary electrodes, and which further includes an elastomeric member having multiple frusto-conical elastic housings. On an inner surface of the top wall of each elastic housing, there is bonded a movable electrode which is disposed opposite to the corresponding pair of stationary electrodes. This printed circuit board is disposed below the key holder plate so that key switches comprising the keys and the movable and stationary electrodes are operated through depression of the keys in a known manner. To establish a curved operating surface of the top faces of the keys, a curved retainer plate made of metal is used. This retainer plate is formed with a predetermined curvature in cross section along a line perpendicular to the rows of the keys. The key holder plate and the printed circuit board are placed on the curved retainer plate. In this condition, the retainer plate and the printed circuit board are fixed to the upper casing with one set of screws, and the retainer plate is further secured to the upper casing with another set of screws. With the retainer plate fixed to the upper casing, the printed circuit board and the key holder plate are held curved along the curvature of the curved retainer plate, and along a convex lower end profile of downward extensions which are provided on the lower surface of the upper casing on both right and left sides of the rectangular aperture.
Thus, the operating surface of the key top faces of the above proposed keyboard is downwardly curved for easy operation of the keys. This curvature is obtained without forming the keys in different sizes and shapes depending upon their positions on the key holder plate. Further, the fabrication of a planar key holder plate with integral guide portions of the proposed keyboard is easier than the traditional fabrication of a curved key holder plate with integral guide portions by molding of a synthetic resin material. In molding such a curved key holder plate with the guide portions, there is a problem of mold release because the guide portions are formed in radial directions.
However, the above arrangement of the keyboard proposed in U.S. Pat. No. 4,528,428 requires a precise positioning of the upper casing, key holder plate, printed circuit board and curved retainer plate, with respect to each other. In particular, the key holder plate should be accurately positioned relative to the downward extensions on the sides of the rectangular aperture. This positioning is relatively difficult and troublesome, because the components are secured to the upper casing by using many screws at different locations.
Further, the above arrangement requires the printed circuit board to be be curved while the curved retainer plate is fixed to the upper casing with screws. That is, the key holder plate is curved through deformation of the printed circuit board while the retainer plate is fixed to the upper casing. This method leaves a possibility of incomplete flexure or deformation of the printed circuit board, which results in the failure of the key holder plate to be given an intended curvature in conformity with the curvature of the curved retainer plate.
The use of a curved retainer plate of metal in the above proposed arrangement is advantageous for permanency of its initial curvature created through plastic deformation thereof, i.e., highly capable of maintaining the designed curvature of the operating surface of the keys. However, this metal plate is required to be relatively thick for permanency of its shape, thereby causing an increase in total weight and depth of the keyboard assembly.